As a method for drilling processing of a laminated board or a multi-layer board used in a printed wiring board material, a method has been generally adopted including piling one or more laminated boards or multi-layer boards, arranging, as an entry board, an aluminum foil alone or a sheet obtained by forming a resin composition layer on a surface of the aluminum foil (hereinafter, the “sheet” is referred to as “entry sheet for drilling”) on the top of the laminated board or multi-layer board, and conducting hole boring processing. Although generally a “copper clad laminate” is often used as the laminated board, the laminated board may be a “laminated board that does not have copper foil on an outer layer”. In the present invention, the laminated board refers to the “copper clad laminate” and/or the “laminated board that does not have copper foil on an outer layer” unless otherwise stated.
In recent years, improvements in reliability are demanded for printed wiring boards, and the high densification of the printed wiring boards progresses, high-quality drilling processing including the improvements in hole position accuracy and the reduction in roughness of hole walls has been demanded for the drilling processing of a laminated board or a multi-layer board.
To meet above-described demands such as the improvements in hole position accuracy and the reduction in roughness of hole walls, a method for hole boring processing using a sheet containing a water-soluble resin such as a polyethylene glycol is proposed in Patent Literature 1. Moreover, a lubricant sheet for hole boring obtained by forming a water-soluble resin layer on a metallic foil is proposed in Patent Literature 2. Further, an entry sheet for hole boring obtained by forming a water-soluble resin layer on aluminum foil provided with a thin film of a thermosetting resin formed thereon is proposed in Patent Literature 3. Furthermore, a lubricant sheet for hole boring in which a non-halogen colorant is blended in a lubricating resin composition is proposed in Patent Literature 4.
As one example of the entry sheet for drilling, an entry sheet is proposed comprising a metallic foil and a layer of a resin composition, the layer formed on at least one surface of the metallic foil. However, the adhesion strength between the metallic foil and the layer of the resin composition is weak, and therefore the layer of the resin composition is peeled during drilling processing in the configuration of the entry sheet for drilling in which the metallic foil and the layer of the resin composition are in direct contact with each other, and the drill steps on the peeled layer of the resin composition, resulting in the deterioration of the hole position accuracy and the increase in the frequency of breakage of the drill in many cases. Moreover, the entry sheet for drilling is usually used in hole boring processing in such a way that the entry sheet is arranged on both surfaces of a plurality of laminated boards or multi-layer boards to make a set with a tape for fixation; however, the tape for fixation may be peeled together with the layer of the resin composition and the position of the entry sheet may be shifted. Therefore, the entry sheet for drilling practically used includes an adhesion layer (adhesion film) containing a urethane-based compound, a vinyl acetate-based compound, a vinyl chloride-based compound, a polyester-based compound, a copolymerized product thereof, an epoxy-based compound, a cyanate-based compound, or the like formed between the metallic foil and the layer of the resin composition in order to improve the adhesion strength between the metallic foil and the layer of the resin composition (see, for example, Patent Literature 5).